Electrical machines need cooling to dissipate the heat, which is generated during the operation of the machine. The heat is generated by ohmic resistance, iron hysteresis, etc.
It is possible to cool a small electrical machine by a heat-transfer from the inside of the machine to its surface. This is not possible for a large machine, which shows a relatively small surface per power rating and heat generation.
When a machine is installed indoors (in a dry atmosphere) it is possible to operate the machine without housing. The cooling of the machine is achieved by a circulation of ambient air through the machine.
If the electrical machine is installed under harsh conditions, for example when the generator is used in an offshore wind turbine, the electrical machine needs to be totally enclosed. Salty ambient air must not be allowed to circulate through the machine. For this specific situation a dedicated system for cooling is required.
One very common cooling method is the circulation of air or another gaseous medium inside the electrical machine, while the cooling medium is kept cool by a heat exchanger. The cooling system for a large electrical machine, like a wind-turbine-generator, becomes big and expensive when air cooling is used.
A more cost effective design is achieved, if a liquid cooling system is used. However it is difficult to implement it in a robust and efficient design. The reason is that a number of cooling channels need to be integrated or attached to the generator near the areas, where the heat is generated.
The European patent-application EP 08 018 802 show a cooling arrangement for a generator. The generator contains a stator, which shows a number of stacked laminate plates. The laminate plates carry on a first side a number of stator coils, while the stator coils interact with a rotor of the generator. The first side is opposite an air gap, which is between the stator and the rotor. There is a fixing-arrangement, which is used to fix the stacked laminate-plates. The fixing-arrangement is located on a second side of the laminate plates, while this second side is opposite the first side. Heat is generated near the stator coils and within the laminate plates as described above. To transfer the heat the fixing-arrangement is designed to be used as a tube for a cooling-liquid. So the generated heat is transferred from the metal-windings of the stator coil and the laminate-plates to the cooling-medium by heat-conduction.
This arrangement shows a disadvantage—it is very difficult—to ensure a good thermal-contact between the laminate-plates and the tubes, which are used for the cooling.
A thermal-expansion between the laminate-plates and the cooling-channels may lead to small air-gaps between the laminate plates and the cooling-channels, so the heat-transfer is affected negatively.
US 2007 024 132 A1 describes a wind turbine generator. The generator includes a stator with a core and a plurality of stator windings. The windings are mounted in a circumferentially spaced manner around the longitudinal axis of the generator.
A rotor rotates around the generator's longitudinal axis. The rotor includes a plurality of magnetic elements coupled to the rotor. The magnetic elements work in conjunction with the stator windings, to generate power by the magnetic field in the stator windings.
There is a heat pipe assembly, which is thermally engaged with the stator and the rotor to dissipate the generated heat. It is very difficult to arrange the cooling pipes inside the stator, especially in a setup of a production.
It is even more difficult to handle the cooling-arrangement in a repair situation. Furthermore, it is difficult to position the cooling channels inside the stator and at the same time to ensure a film and an effective thermal contact.